(1) Field of the Invention
The present invention relates to an image forming device for achieving image formation by transporting a recording sheet with a toner image having been transferred thereon so that the toner image is thermally fixed onto the recording sheet, and to an image forming method for use in the image forming device.
(2) Description of the Related Art
There has been proposed a technology in which an image forming device is provided with a sensor that detects a temperature of a fixing roller for a temperature adjustment control during a warm-up such that the image forming device can start executing the image forming operation, the sensor also detects a temperature of the recording sheet after the fixing, and it is determined whether an error has occurred, based on the temperatures of the fixing roller and the recording sheet detected by the sensor (Patent Document 1: Japanese Patent Application Publication No. 10-161468). More specifically, when there is an extreme difference between the detected temperatures of the fixing roller and the recording sheet, it is determined that an error has occurred.
There has also been proposed a technology in which an image forming device is provided with a temperature sensor at a position more downstream in a recording sheet transport direction than a fixing nip, which is formed between a fixing roller and a pressurizing roller, where the temperature sensor detects the temperature of the fixing roller facing the temperature sensor with a recording sheet transport route running therebetween (Patent Document 2: Japanese Patent Application Publication No. 10-198216). When the recording sheet passes between the fixing roller and the temperature sensor, the measuring area of the temperature sensor is blocked by the recording sheet, and the temperature sensor detects the temperature of the recording sheet. This makes it possible to determine whether or not a fixing error occurred, based on the detected temperature of the recording sheet.
Meanwhile, it is desired that the image forming device has simplified wiring and less number of constituent parts.
Also, for the sake of user's convenience, the image forming device needs to be heated rapidly when it is powered on. This may be accomplished by reducing the thermal capacity of the fixing roller. However, when the thermal capacity of the fixing roller is reduced, the temperature thereof is easy to change. In that case, to stably fix the image onto the recording sheet, the temperature adjustment needs to be controlled more carefully than conventionally.
The technology of the Patent Document 2 contributes to the reduction in the number of constituent parts and simplified wiring because with this technology, only one temperature sensor is used to detect the temperatures of the fixing roller and the recording sheet. The Patent Document 2, however, does not have any recitation concerning the temperature adjustment control of the fixing roller.
Suppose here that the temperature adjustment control performed during the warm-up as disclosed in the Patent Document 1 is applied to the technology of the Patent Document 2. Then, it would be possible to perform the temperature adjustment control during the warm-up, based on the temperature of the fixing roller.
Such an image forming device would be able to detect the temperature of the fixing roller while a recording sheet is not passing between the fixing roller and the temperature sensor during the image forming operation, but would not be able to detect the temperature of the fixing roller while a recording sheet is passing. In such an image forming device, the temperature adjustment control would be available during the warm-up, but a careful and detailed temperature adjustment control would not be available.